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La spectroscopie à transformée de Fourier
Le FTS de Herschel-SPIRE et ses potentialités scientifiques
Kjetil Dohlen
Plan de l’exposée
• Objectifs et potentialites scientifiques
• Vue globale de l’instrument– L’évolution de l’instrument
– Pourquoi un FTS
• Les bases du FTS
• Les performances de l’instrument– Résolution spectrale
– Echantillonnage et résolution spatiale
• Expérience personnelle
Le télescope Herschel• Télescope de 3.5m diamètre
– 3.3m pupille entrée
• Lancé en 2007 par Ariane 5
• Orbite autour de L2
• Trois instruments montes dans le cryostat– HiFi: Spectrométrie
hétérodyne, 156-625 µm
– PACS: Imagerie et spectroimagerie 60-210 µm
– SPIRE: Imagerie et spectroimagerie 200-670 µm
• Refroidies par 2000 litres de He liquide– Plus que 3 ans opération
Les instruments de Herschel
Solar system: giant planets, comets and
solid bodies
Star formation and interstellar matter
Statistics and physics of galaxy formation in
the early universe
Galaxies – normal, starburst and AGN
SPIRE Scientific Goals
PA
CS
SP
IRE
1001000 10 (m)
Flu
x d
ensi
ty (
Jy)
Flu
x d
ensi
ty (
Jy) 10
1
0.1
0.01
10 100 1000
1012L
0.5
5
3
1
Z = 0.1
(m)
5, 1h
R=40
R=3
Potentialités du FTS
• Spectre complet dans le domaine 200 – 670 m
• Resolution variable– Raies, R=1000
– Continu, R=40
• Spectro-imagerie – Imagerie dans des raies (atomiques et moléculaires)
– Etude des conditions physiques dans différents milieux
• Large domaine spectral – Possibilité d’observer plusieurs transitions d’une même molécule
– Etude des conditions physiques
• Photometer- Deep mapping with highest efficiency and largest possible field of view
- Multi-band coverage with simultaneous observation - Point and compact source observation with high efficiency
• Spectrometer - Sensitivity optimised for point/compact source spectroscopy - Imaging spectroscopy with maximum available field of view - Wide wavelength coverage
- Variable spectral resolution (few x 10 to few x 100)
• Both - Thermal background dominated by the Herschel telescope - Simplicity, affordability, reliability, ease of operation - Complementary to other Herschel instruments and other facilities
Instrument Design Drivers
SPIRE Focal Plane Unit
Photometerside
Spectrometerside
690 mm
Thermally isolating supports
Central optical bench panel
2-K thermal straps
Light-tight baffles at strap
entry points
3He cooler
Photometer Layout and Optics
Herschel focal
surface
2-Kcoldstop
M3
M4
M5
M6
M7
M8
Beam steering mirror
Offner relay
Dichroics and
arrays
M9
Detector arraymodules
Beam steering sirror
SPIRE optical bench (4 K)
2-K box
M3
M4
M5M7
M6M8
FTS Layout and Optics
Telescope input port
Calibrator input portOutput
port
Output port
Intensity beam
dividers
Fore-optics shared with photometer
Mirrormechanism 2nd-port
calibrator
Beam divider
4-Kbox
Baffle
2-Kbox
Detector array
modules
Evolution of the instrument (1)
• Original proposal for the BOL instrument:– Double Fabry-Perot– Abandoned because of its
design complexity
• February 1997– Separation of photometer and
spectrometer channels• « SpecBOL »• « PhotBOL »
– Scanning flat grating spectrometer working in multiple orders
– Included lenses
Evolution of the instrument (2)• March 1997:
– All-reflective flat grating design
• June 1997– Prospect of bolometric array
detectors
– Study of static, all-reflective cross-dispersed design
• Concave « holographic » main grating
• Offner-type concentric cross-dispersion spectrograph
Evolution of the instrument (3)• November 1997: The ultimate
grating design– Concave « holographic » grating
– Reimaged pupil for cold stop
– Simultaneous detection in several orders, allowing:
• sufficient wavelength range with limited grating scan range
• multiplex advantage
– Advantage for LAM:• ISO-LWS heritage for grating
mechanism
– Problems:• R ~ few 100
• No imagery
• Extremely stray light sensitive
2 K
B
R1
R2
R3
G
C
4 K
D
Spurious order
PO
B
CS
S
GM
S’
Evolution of the instrument (3)• November 1997: The ultimate
grating design– Concave « holographic » grating
– Reimaged pupil for cold stop
– Simultaneous detection in several orders, allowing:
• sufficient wavelength range with limited grating scan range
• multiplex advantage
– Advantage for LAM:• ISO-LWS heritage for grating
mechanism
– Problems:• R ~ few 100
• No imagery
• Extremely stray light sensitive
2 K
B
R1
R2
R3
G
C
4 K
D
Spurious order
PO
B
CS
S
GM
S’
« Mais alors, pourquoi pas un FTS ? »
• R~1000 possible
• Imagerie
• Bande continue
• Moins sensible a la lumière parasite
Et une nouvelle chasse aux designs commença...
Evolution of the instrument (4)• December 1997: SWIFT
– Swinging arms FTS
– Martin-Puplett polarized design
– Advantage for LAM:• ISO-LWS heritage for mechanism
– Retained for the ESA proposal
– Problems:• R~500
• 50% maximum efficiency
D1F1
D2
F2
P3
P2 RT2
RT1
P1
BB
Evolution of the instrument (5)• October 1998: Polarizing
Mach-Zehnder– Martin-Puplett polarized design
with dual inputs and outputs
– Potentially 100% efficiency
– Up to R~1000
– Problems:• Extremely cumbersome
• Difficult alignment
• No ISO-LWS heritage for mechanism
– Mechanism concept from GSFC proposed
Evolution of the instrument (6)• February 1999: Mach-Zehnder
with 50/50 beam splitter– Wide-band beamsplitter developed
by QMW (P. Ade)• Metal-mesh filter technology
– Much more compact• No more need for input and
output polarizers
– Potentially 100% efficiency
– Up to R~1000
– Problems:• What problems?
Evolution of the instrument (7)• Ah-oui, le mécanisme, fallait
quand-même le faire...– Développement sous
responsabilité LAM (Pascal Dargent et al.)
– Principe GSFC retenu• Course• Stabilité
– Modifications importantes• Passage du faisceau• Masse• Tenu vibrations
Evolution of the instrument (8)• Pour ne pas parler du contrôle
commande– Développement sous responsabilité
LAM (Didier Ferrand et al.)
– Senseur de position Heidenhein• Spatialisé avec ObsPM (G. Michel)
et CEA
Comment ça marche, un FTS ?
0
0.25
0.5
0.75
1
0 5 10 15 20 25
Frequency
Sp
ectr
um
• Interférogramme = FT(Spectre)– Une ligne en émission = Signal en Cosinus
– Spectre large = Somme de Cosinus
• Spectre = FT(Interférogramme)
0
0.25
0.5
0.75
1
-15 -10 -5 0 5 10 15
Optical path difference
Inte
rfer
og
ram
0
0.2
0.4
0.6
0.8
1
1.2
-6 -4 -2 0 2 4 6
Optical path difference
Inte
nsi
ty
Résolution d’un FTS• La résolution d’un FTS est définie comme
R = ou = 1/est le « nombre d’onde », proportionnel à la fréquence
• Plus l’interférogramme est long (OPD grand), plus la résolution spectrale ( est fine:
= 1/(2 OPD)
• On a donc que:
R = 2 OPD/• Alors que dans le cas
d’un réseau limité par la fente:
Rslit • Pour SPIRE
– OPD = 4*course = 4*31.25mm = 125mm
– Pour = 250m, on a donc
R250m = 1000
0
200
400
600
800
1000
1200
1400
200 300 400 500 600 700
Wavelength (m)
Res
olv
ing
po
wer
Resolution maximale de SPIRE
Detector Arrays (2F Feedhorns)
45 mm
PLW43 detectors
PMW88 detectors
22 mm
SLW19 detectors
SSW37 detectors
Photometer Spectrometer
Coincident beam centres
PSW139 detectors
200-315 m
315-670 m
500 m 350 m 250 m
PLW Array
Image sampling• Gaussian mode feedhorn detectors
– PSF on the sky has ~ Gaussian profile
– FWHM ~ /D, slightly broader than Airy profile
• Pixels separated by 2/D– Image is undersampled
– « Jiggling » of the image required
– 16 pointings for full sampling
PLW Array
FTS Observing Modes• = 0.04 - 2 cm-1 (R250m = 1000 - 20) by adjusting scan length
• Continuous scan:- Mirror scan rate = 0.5 mm s-1
- Signal frequency range = 3 - 10 Hz - Calibrator in 2nd port nulls telescope background
• Step-and-integrate:- 2nd port calibrator is off
- Mirror stepped with integration at each position - BSM chops on sky
• Imaging spectroscopy- Beam steering mirror adjusts pointing between
scans to acquire fully-sampled spectral image
• Point source spectroscopy/spectrophotometry- Telescope pointing fixed- Background characterised by adjacent pixels
200 300 400 500 600 7000
100
200
300
400
500
Wavelength (microns)
Lim
itin
g lf
lux
den
sity
(m
Jy)
a
Sensitivity Estimates: Spectrometer
200 300 400 500 600 7000
5
10
15
20
Wavelength (microns)
Lim
itin
g lin
e fl
ux
(w m
-2 x
1E
-17)
a
(m)Lin
e fl
ux
5
1 h
r (W
m-2 x
1E-1
7)
Map
Point source
Line spectroscopy( = 0.04 cm-1)
Flu
x d
ensi
ty 5
1
hr
(mJy
)
(m)
Map
Point source
Low-resolution spectrophotometry
( = 1 cm-1)
R250 = 1000 R250 = 40
Science avec le FTS une expérience personnelle
• FTS statique – HFTS: Holographic FTS
– HHS: Heterodyne holographic spectrometer
– SHS: Static heterodyne spectrometer
– ...
• Compact, portable– Environnement
– Atmosphère
– Végétation
– Géologie
• Avantage de l’étendue– Absence de fente
Science avec le FTS NO2 dans l’air de Londres
Model simplifié de expérience
NO2 observé dans l’atmosphère
NO2 observé en laboratoire
7 x 10-6 estimé
Les raies de Fraunhofer permettent de calibrer
(H)
(Fe)
Peak
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